Conventional optical discs for disc recorders or players must provide an ideal plane in order for an incident laser beam to be always perpendicular to the disc surface. However, in real world applications, such optical discs can have mechanical distortion that creates an angular deviation (or tilt) with respect to the laser beam. The deviation angle (represented by the Greek letter THETA) is generally referred to as disc tilt. Disc tilt affects disc read back by degrading signal quality and enlarging the data error rate. In the case of disc recording, tilted recording data is hard to read back by other players without a similar tilt, or with a different tilt pattern. Disc tilt changes from disc to disc. Disc tilt can change even from zone to zone within the same disc. To compensate for disc tilt changes, some kind of tilt servo control is required to eliminate the problems caused by the disc tilt.
Normally, disc tilt is so tiny that detection is not easy. One way to determine disc tilt involves detecting or comparing signal amplitude or intensity. The signals involved can include a photo diode signal (RF), a tracking error signal (TE), a focus error signal (FE), wobble or even. a signal generated by differential phase detection (DPD) or differential push-pull (DPP) techniques. Conventional solutions that detect or compare signal amplitude or intensity to determine disc tilt can be found in U.S. Pat. Nos. 6,690,632; 6,625,093; 6,587,409; 6,577,568; 6,549,493; 6,493,296; 6,169,715 and 6,459,664.
Conventional signal detection and comparison methods, no matter which signal is used, tend to be less effective in real applications. Signal deviation caused by factors other than disc tilt can be larger than the deviation caused by the disc tilt. The tilt angle belongs to a geometry catalog. The signal deviation is a composite result caused by multiple factors including tilt. Any signal deviation observed is not a root cause of tilt, nor a geometric measurement. Although signal deviation does change with tilt, decoupling the component caused by disc tilt from multiple other components caused by other factors is very difficult. Thus, any method dealing with signal deviation cannot provide an ultimate solution.
Other conventional solutions involve using (i) a dedicated photo sensor for disc tilt detection (e.g., U.S. Pat. Nos. 6,434,096 and 6,430,130 implement such an approach) or (ii) an additional optical detection system with independent photo-electronics and an overlapped optical path (e.g., U.S. Pat. No. 6,735,157). Although dedicated photo sensors have been used in commercial products, such systems have problems such as (i) high cost, (ii) low resolution, (iii) extra power consumption and (iv) beam position error because the sensing beam is always away from the main laser beam. Furthermore, a dedicated photo sensor needs extra calibration procedures to provide information that can be trusted. A misaligned sensor is even worse than no tilt control.